feat: self-service sign-up with email verification and onboarding

Complete sign-up pipeline: email registration via Logto Experience API,
SMTP email verification, and self-service trial tenant creation.

Layer 1 — Logto config:
- Bootstrap Phase 8b: SMTP email connector with branded HTML templates
- Bootstrap Phase 8c: enable SignInAndRegister (email+password sign-up)
- Dockerfile installs official Logto connectors (ensures SMTP available)
- SMTP env vars in docker-compose, installer templates, .env.example

Layer 2 — Experience API (ui/sign-in/experience-api.ts):
- Registration flow: initRegistration → sendVerificationCode → verifyCode
  → addProfile (password) → identifyUser → submit
- Sign-in auto-detects email vs username identifier

Layer 3 — Custom sign-in UI (ui/sign-in/SignInPage.tsx):
- Three-mode state machine: signIn / register / verifyCode
- Reads first_screen=register from URL query params
- Toggle links between sign-in and register views

Layer 4 — Post-registration onboarding:
- OnboardingService: reuses VendorTenantService.createAndProvision(),
  adds calling user to Logto org as owner, enforces one trial per user
- OnboardingController: POST /api/onboarding/tenant (authenticated only)
- OnboardingPage.tsx: org name + auto-slug form
- LandingRedirect: detects zero orgs → redirects to /onboarding
- RegisterPage.tsx: /platform/register initiates OIDC with firstScreen

Installers (install.sh + install.ps1):
- Both prompt for SMTP config in SaaS mode
- CLI args, env var capture, cameleer.conf persistence

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/sign-in/Dockerfile

@@ -15,6 +15,9 @@ FROM ghcr.io/logto-io/logto:latest
 # Install bootstrap dependencies (curl, jq for API calls; postgresql16-client for DB reads)
 RUN apk add --no-cache curl jq postgresql16-client
 
+# Install all official Logto connectors (ensures SMTP email is available for self-hosted)
+RUN cd /etc/logto/packages/core && npm run cli connector add -- --official 2>/dev/null || true
+
 # Custom sign-in UI
 COPY --from=build /ui/dist/ /etc/logto/packages/experience/dist/